This invention relates to telecommunications systems and, more particularly, to a method and apparatus for accessing a system utilizing an access channel providing reduced access delay in a telecommunications system.
Major cellular telecommunications systems types include those operating according to the Global Services for Mobile (GSM) Standard, the TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual Mode Wide Band Spread Spectrum Cellular Systems (IS-95A, currently being updated as IS-95B in the document TIA/EIA SP-3693), the TIA/EINIS-136 Mobile Station-Base Station Compatibility Standard (IS-136), and the TIA/EIA 553 Analog Standard (AMPS/TACS). Other major cellular systems include those operating in the personal communications system (PCS) band according to the IS-95 based ANSI-J-STD-008 1.8-2.0 GHz standard or those operating according to the GSM-based PCS 1900/1910 MHz frequency range standard. Currently, each of the major cellular system standards bodies is implementing packet data services into its digital cellular specifications. A packet data specification has been finalized for GSM and IS-95A. Packet data specifications compatible with the IS-136 and IS-95B standards are also being prepared.
In a typical cellular system a call establishment begins either by a base station transmitting a paging message to a mobile station on a paging channel and then the mobile station transmitting a paging response message to the base station on an access channel, or by a mobile station accessing the system on an access channel by transmitting an origination message to a base station. In either of these call establishment cases, the mobile station must access the system on an access channel, and information unique to the particular call establishment must be exchanged between the mobile station and base station over the access channel or other channels of the system air interface. The paging response message and origination message typically carry a large portion of the information. The information unique to the particular call establishment could include called number data, mobile station identification and capability related data, authentication information, etc. After receiving this information, the system must then use the information to set up the different layers of communication necessary in the system to implement the call.
In packet data applications, a mobile station establishes a connection with the base station when it has one or more data packets in the buffer of the mobile station to send or when it is paged by a base station having data packets to send. The mobile station accesses the system for a channel connection and transmits until it is determined that no data exists in the buffer for transmission. Since data may be received from a data server at the mobile or base station intermittently, it may be necessary to release the channel connection in order to maximize the use of the channel by other mobile stations. This means that the mobile station will be making multiple access attempts to establish a channel connection, each access attempt being made when the mobile or base station has enough data to transmit. Each access attempt may in itself involve more than one access attempt if initial access attempts are unsuccessful. In the current packet data system for GSM, IS-95A, IS-95B and IS-136, the access channel has a fixed frame size and data rate. For example, the IS-95B packet data access channel is the same channel used to originate regular calls. The IS-95B access channel has a frame size of 20 msec and a data rate of 4.8 kbps.
As third generation systems which will replace GSM, IS-136 and IS-95B are developed and packet data usage becomes more common, solutions must be found to handle packet data service delays that may be caused by the delays incurred when requesting access to the system each time packet data is to be sent. If many packet data users are in the system competing for channels, there will be a need to release access channels as often as possible and a need to perform new accesses following release of the accessed channel if new data is accumulated for transmission. An improved access procedure will be required for packet data services in these systems.
It is, therefore, an object of this invention to provide an improved method and apparatus for accessing a telecommunications system that overcomes the foregoing and other problems.
Another object of this invention is to provide a method and apparatus for accessing a telecommunications system using a channel providing reduced access delay.
Another object of this invention is to provide a method and apparatus for accessing a telecommunications system using a channel having variable data rates and frame sizes for access.
A further object of this invention is to provide a method and apparatus for accessing a telecommunications system using a channel having variable data rates and frame sizes assignable to a mobile station based on channel conditions and service type required.
The present invention provides an improved method and apparatus for accessing a telecommunications system. The method and apparatus utilizes a channel having variable data rates and frame sizes. In the method and apparatus, access to the system may be requested via at least one channel having variable data rates. Each of the available data rates is associated with at least one transmission frame size of a plurality of frame sizes. If channel conditions allow, a higher data rate of the available data rates may be used to request access over the channel. The method and apparatus has an advantage for use in packet data services. By dynamically determining the data rate based on channel conditions, overall access delays for mobile stations using packet data services and making many access attempts may be reduced.
In an embodiment of the invention, a plurality of access channel data rates and frame durations are available for use by a mobile station requesting access. The data rates and frame durations may be set so that the number of data bits per frame is constant for ease of processing. A mobile station accessing the system selects a data rate and associated frame duration based on channel conditions, mobile station power conditions or the type of service required. Packet data service users requiring shorter access delay may select a higher data rate and associated frame duration for a particular type of service under certain channel conditions, subject to transmission power requirements. Since a higher data rate requires a higher transmitted power to achieve a comparable Eb/No as at a lower data rate, transmission power of the mobile station at the higher rate must be increased compared to that at the lower rate without exceeding the maximum allowable transmitted power for the mobile station so the frame error rate (FER) and bit error rate (BER) remain within acceptable limits.
Transmission power requirements may be determined on the basis of a desired Eb/No to be received at the base station antenna for access attempts by the mobile station. The mobile station determines whether an estimated path loss is less than a maximum allowable path loss for a desired data rate for access. If the estimated path loss is less than the maximum allowable path loss at the desired data rate for access and the mobile station transmission power necessary to achieve the desired Eb/No does not exceed the maximum allowed power the mobile station is limited to for acceptable system performance, the desired data rate is selected. If the estimated path loss is greater than the maximum allowable path loss for the desired data rate for access or the necessary mobile station transmission power exceeds the allowable maximum for the mobile station, a lower data rate having a maximum allowable path loss greater than the estimated path loss and/or greater than the maximum allowed power for the mobile station is selected.